(1) Field of the Invention
The present invention relates generally to communications antennas, and more particularly to a plasma antenna for High Frequency (HF) communications.
(2) Description of the Prior Art
Current communication methods for underwater environments include the use of mast mounted antennas, towed buoys, and towed submersed arrays. While each of these methods has merits, each presents problems for use in an underwater environment. The mast of current underwater vehicles performs numerous sensing and optical functions. Mast mounted antenna systems occupy valuable space on the mast which could be used for other purposes. For both towed buoys and towed submersed arrays, speed must be decreased to operate the equipment.
Plasma antennas are of interest for communications with underwater vessels since the frequency, pattern and magnitude of the radiated signals are proportional to the rate at which the ions and electrons are displaced. The displacement and hence the radiated signal can be controlled by a number of factors including plasma density, tube geometry, gas type, current distribution, applied magnetic field and applied current. This allows the antenna to be physically small, in comparison with traditional antennas. Studies have been performed for characterizing electromagnetic wave propagation in plasmas. Therefore, the basic concepts, albeit for significantly different applications, have been investigated. These efforts have included a Corona Mode antenna that utilizes the corona discharges of a long wire to radiate ELF signals, a propane plasma antenna, and studies of electromagnetic propagation in plasmas. Other research has focused on characterizing the electromagnetic waves that exist in plasmas. In addition, U.S. Pat. No. 3,914,766 to Moore discloses a pulsating plasma antenna which has a cylindrical plasma column and a pair of field exciter members parallel to the column. The location and shape of the exciters, combined with the cylindrical configuration and natural resonant frequency of the plasma column, enhance the natural resonant frequency of the plasma column, enhance the energy transfer and stabilize the motion of the plasma so as to prevent unwanted oscillations and unwanted plasma waves from destroying the plasma confinement. However, as configured, the Moore antenna lacks the capability of being electronically steered and dynamically reconfigured. Such steering and reconfiguration would allow the antenna to be more efficient and operate in a wider band of frequencies. U.S. Pat. No. 5,594,456 to Noris et al. discloses an antenna device for transmitting a short pulse duration signal of predetermined radio frequency that includes a gas filled tube, a voltage source for developing an electrically conductive path along a length of the tube which corresponds to a resonant wavelength multiple of the predetermined radio frequency and a signal transmission source coupled to the tube which supplies the radio frequency signal. The antenna transmits the short pulse duration signal in a manner that eliminates a trailing antenna resonance signal. However, as with the Moore antenna, the band of frequencies at which the antenna operates is limited since the tube length is a function of the radiated signal.